Recent developments have allowed local device-to-device (D2D) communication to (re)use cellular spectrum resources simultaneously with ongoing cellular traffic. In general, non-orthogonal resource sharing between cellular and D2D layers has the potential of reuse gain and proximity gain, while at the same time increasing resource utilization. Consequently, D2D communications underlying cellular networks have received considerable interest in recent years.
In 3GPP Long Term Evolution (LTE) networks, such LTE Direct (D2D) communication can be used in commercial applications, such as cellular network offloading, proximity based social networking, or in public safety situations in which first responders need to communicate with each other and with people in a disaster area (per 3GPP TR 22.803).
FIG. 1 is a diagram illustrating an example LTE network. A communication network 100 comprises a plurality of wireless devices 110 (e.g., conventional user equipment (UEs), machine type communication (MTC)/machine-to-machine (M2M) UEs) and a plurality of network nodes 115 (e.g., radio access nodes such as eNodeBs (eNBs) or other base stations). Communication network 100 is organized into cells 125, which are connected to core network 120 via corresponding network nodes 115. Network nodes 115 are capable of communicating with wireless devices 110 along with any additional elements suitable to support communication between wireless devices 110 or between a wireless device 110 and another communication device (such as a landline telephone).
D2D communication entities using an LTE Direct link may reuse the same physical resource blocks (PRBs) (i.e., time/frequency resources) used for cellular communications either in the downlink (DL), in the uplink (UL), or both. The reuse of radio resources in a controlled fashion can lead to the increase of spectral efficiency at the expense of some increase of the intra-cell interference.
Typically, D2D communicating entities (e.g., wireless devices 110) use UL resources such as UL PRBs or UL time slots, but conceptually it is possible that D2D (LTE Direct) communications take place in the cellular DL spectrum or in DL time slots. For convenience, the following description presents various embodiments in which D2D links use UL resources, such as UL PRBs in a Frequency-Division-Duplex (FDD) or UL time slots in a cellular Time-Division-Duplex (TDD) system, but the described concepts are applicable to cases in which D2D communications take place in DL spectrum as well.
In certain scenarios, network node 115 (for example, a control node such as an eNB) assigns resources to each wireless device 110 for D2D operation, and in other scenarios wireless devices 110 control directly at least some of the transmission resources used for D2D transmission. Distributed resource allocation schemes may be used in the latter case, with reduced signaling and dependency on network node 115 as compared to centralized resource allocation. In distributed schemes, wireless devices 110 are typically constrained to use resources belonging to a possibly periodic resource pool, typically consisting of a set of time and frequency resources.
The 3GPP standardization body defines two different types of services for D2D communications: (1) Direct Communication where wireless devices 110 in proximity of each other establish a direct user plane connection; and (2) Direct Discovery where wireless devices 110 transmit and monitor discovery announcement to become aware of the type of content/service each wireless device 110 can share in D2D fashion, as well as the proximity between each other. When performing direct communications, 3GPP defines two different operative modes: (1) Mode 1, in which a wireless device 110 in RRC_CONNECTED mode requests D2D resources and network node 115 grants them; and (2) Mode 2, where a wireless device 110 (potentially in RRC_IDLE) autonomously selects resources for transmission.
Similarly, for direct discovery, 3GPP considers Type 1 in which wireless devices 110 autonomously select radio resources for discovery, and Type 2 in which wireless devices 110 in RRC CONNECTED request resources for discovery and network node 115 grants them via radio resource control (RRC).
D2D communication within LTE should be able to work also inter-Public-Land-Mobile-Network (PLMN) (as well as intra-PLMN both inter-frequency and intra-frequency). Therefore, according to one example a wireless device 110 operating under a first operator subscription on a first carrier frequency should be able to discover (and in a later stage also communicate to) a second wireless device 110 operating under a second operator subscription on a second carrier frequency.
The performance of multi-carrier D2D wireless devices 110 may be improved in different ways in terms of discovery latency, discovery probability and energy efficiency. With multi-carrier D2D, a configuration is intended where a wireless device 110 performs D2D transmission and/or reception on one carrier while it is also configured to perform cellular or other communications (including other D2D communications) on another carrier. Such carriers may or may not be managed by the same network node 115 and/or network operator. Additionally, such carriers may also be specially preconfigured carriers that wireless device 110 monitors but that are not directly managed by any network infrastructure, meaning that the network cannot provide system information for that carrier or assign transmission resources. These carriers are referred to as out-of-coverage carriers, and wireless device 110 has to autonomously select transmission resources (Mode 1/Type 2) from a resource pool in order to transmit.
Potential benefits of multi-carrier D2D communications are manifold. For example, because D2D communication exploits the ordinary cellular spectrum, it may be important to better exploit available carriers to limit the impact on the quality of service of the cellular layer. This feature could allow a better load distribution among carriers and eventually an increased quality of service (QoS) for both cellular and D2D communications. Additionally, having multiple carriers available for D2D communications widens the range of services/applications that the D2D technology can satisfy (e.g., it gives the possibility to deploy dedicated carriers for specific services).